The present invention relates to a particulate trap for trapping and removing particulates such as carbon contained in diesel engine exhausts and for removing any noxious components in exhausts.
Exhaust gases from automobiles are a major cause of air pollution. It is therefore of extreme importance to develop a technique for removing noxious components in exhausts.
In particular, the development of a technique for removing particulate components in diesel engine emissions, which are mainly made up of NOx and carbon is urgently required.
To remove such noxious components in exhausts, Unexamined Japanese Patent Publication 58-51235 proposes to provide an exhaust trap in an exhaust line to trap any noxious components and remove them by after-treatment Other conventional exhaust purifying means include various improvements in the engine itself such as exhaust gas recirculation (EGR) systems and improved fuel injection systems. But none of them has proved to be an effective solution. After-treatment methods are currently considered more practical as exhaust purifying means. Thus, rigorous efforts are now being made to develop after-treatment type exhaust purifying systems.
It is required that particulate traps for trapping particulates contained in diesel engine emissions satisfy all of the following requirements.
1) Particulate trapping capability
A particulate trap must be capable of trapping particulates with high efficiency and so that exhausts are satisfactorily purified. It is considered necessary to trap an average of at least 60% of the particulates contained in diesel engine emissions, even though the amount of particulates contained in exhausts depends upon the displacement of the diesel engine and the load applied.
Airborne particulates 2 .mu.m or less in diameter can enter human alveoli and reportedly trigger lung cancer. It is thus necessary that particulate traps be capable of efficiently trapping even these small suspended particulates.
2) Pressure loss
The particulate trap must be capable of keeping the pressure loss in exhaust gases to a minimum If it causes a large pressure loss, a back pressure will act on the engine, aggravating the engine performance in various ways. Thus, it is necessary to keep the pressure loss to less than 30 kPa in a normal state. For this purpose, the particulate trap has to have a sufficiently small initial exhaust pressure loss, and also be capable of keeping the pressure loss to a minimum even after it has trapped a large amount of particulates.
3) Regeneration
The third requirement is that the trap can be regenerated at low energy cost. This is because the particulate trap has to be regenerated or recycled many times for repeated use by burning trapped particulates. An electric heater or a light oil burner is considered a feasible means for burning off particulates.
4) Durability
Fourthly, the trap must be sufficiently durable. It must be highly resistant to corrosion when exposed to hot exhaust gases and to heat shocks produced while burning particulates.
5) Integration with a catalytic converter
Furthers it is necessary to provide a catalytic converter integral with the trap. In order to remove noxious gas components in exhausts, a catalytic converter carrying a noxious gas removing catalyst may be provided in an engine exhaust line. If it is desired to further provide a separate particulate trap in the same exhaust lines there may be no available mounting space in the exhaust line. Also, the cost for separately providing these two different after-treatment type exhaust purifying systems tends to be rather high.
Among the existing filter element materials that satisfy the above-listed requirements, a wall-flow type, honeycomb porous member made of cordierite ceramics is considered the most practical. But this filter has several problems. One problem is that particulates tend to collect locally. Another problem is that, because of low thermal conductivity of cordierite ceramic, this filter tends to develop heat spots during regeneration. As a result, the filter may melt or crack due to thermal stress. Such a filter is not durable enough. A ceramic fiber trap made by forming ceramic fibers into a candle shape is gaining much attention recently. But this trap is not sufficiently durable either. The fibers forming the trap tend to break due to reduced strength when exposed to high-temperature exhaust gases.
Metal traps (as disclosed in Unexamined Japanese Patent Publications 6-257422, 6-294313, 7-731 and 7-51522) are now considered more promising, because they are high in thermal conductivity, less likely to develop heat spots and cracks and highly corrosion-resistant in high-temperature exhausts.
The problems of conventional metal traps in connection with the abovementioned requirements 1)-5) will now be discussed.
Conventional metal traps basically satisfy the requirements 1) and 3). But as to the capacity of trapping suspended particulates 2 .mu.m or less in diameter in the requirement 1), higher performance is desired.
As to the requirement 2), conventional metal traps can cause a marked pressure loss after trapping particulates. If an especially low engine back pressure is required, these traps will be unsatisfactory.
In order to minimize the pressure loss even after the filter element has trapped a large amount of particulates, the filter element must have a large surface area (filtering area). But in order to increase the filtering area of a conventional metal trap filter element, it is necessary to use an extremely large trap.
In connection with the requirement 4): the filter element of a conventional metal trap is deformed microscopically due to the pressure of exhausts introduced, and can-be destroyed due to stresses resulting from the microscopic deformation. Also, since the trap is mounted in the exhaust line, its filter element is vibrated together with the traps which may result in the destruction of the filter element. These problems are observed in harsh endurance tests.
As to the requirement 5), it is sometimes necessary to integrally provide a catalytic converter on a conventional metal trap. For example, a catalyst may be integrally carried on a wall-flow type, honeycomb porous member made of a cordierite ceramic, which was originally developed as a DPF (diesel particulate filter). In such a case, it may be difficult to heat the catalyst to an operational temperature because the honeycomb porous member, having a large heat capacity, is slow to heat up.
An object of the present invention is to provide a particulate trap which is free of all the above problems and which satisfies all the requirements 1)-5).
One way to increase the filtering area of such a metal trap without unduly increasing the size of the entire trap is to weld both sides of all the filter elements to side plates with high accuracy while keeping the filter elements spaced from each other at small intervals. But such accurate welding is technically so difficult that its feasibility is doubtful Even if possible, such welding method would incur prohibitive costs.